1. Field of the Invention
The present invention generally concerns magnetic resonance tomography (MRT) as employed in medicine for examination of patients. The present invention in particular relates to a method and apparatus to reduce distortions or deformations in the plug connection device that appear in the use of echoplanar imaging sequences (EPI sequences) and that significantly negatively affect the image quality (and therefore the diagnosis).
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used in medicine and biophysics as an imaging method for over 15 years. In this examination method the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject which were previously randomly oriented thereby align. Radio-frequency waves can now excite these “ordered” nuclear spins to a precession movement. This precession generates the actual measurement signal in MRT, which measurement signal is acquired by means of suitable acquisition coils. The measurement subject can be spatially coded in all three spatial directions via the use of inhomogeneous magnetic fields generated by gradient sub-coils.
In one method for generation of MRT images, a slice is initially selectively excited, for example in the z-direction. The coding of the spatial information in the slice ensues via a combined phase and frequency coding by means of two orthogonal gradient fields that, in the example of a slice excited in the z-direction, are generated in the x-direction and y-direction by the aforementioned gradient sub-coils. The imaging sequence is repeated M times for different values of the phase coding gradients (for example GP), wherein the nuclear magnetic resonance signal is digitized and stored given each sequence pass in the presence of the readout gradient GR. A number matrix (matrix in k-space) with N×M data points is obtained in this manner. An MR image of the considered slice with a resolution of N×M pixels can be directly reconstructed from this data set.
In imaging known as echoplanar imaging (EPI), multiple phase-coded echoes are used to fill the raw data matrix. The basic idea of this technique is to generate a series of echoes in the readout gradient (GR) after a single (selective) RF excitation, which echoes are associated via a suitable modulation of the phase coding gradient (GP) with different lines in the k-space plane.
One possible form of the echoplanar pulse sequence is shown in FIG. 1. After an excitation pulse and a refocusing pulse, multiple gradient echoes are generated via a sinusoidally oscillating frequency coding gradient in the readout direction and phase coding. The phase coding in this depiction ensues through small gradient pulses (blips) in the range of the zero crossing of the oscillating frequency coding gradient and leads in this manner to a meandering traversal of the spatial frequency matrix (k-matrix), as is shown in FIG. 2. It is thereby noted that, as an alternative to this, EPI can also be implemented as Cartesian EPI (blipped EPI), as non-Cartesian EPI (spiral EPI) or as a single-shot turbo-spin echo (TSE) readout train, for example.
In spite of many limitations, EPI sequences present a high clinical potential (particularly in functional imaging and in perfusion and diffusion measurements) since movement artifacts (for example due to breathing or pulsed movement of blood or cerebral fluid) can be drastically reduced due to the extremely short measurement time (MR image acquisition in less than 100 ms).
As stated, the reason for this is the fact that in EPI an entire 2D MR image is acquired with only a single excitation pulse. A scan direction with high readout speed (readout direction) and a scan direction with low readout speed (phase encoding direction), but results due to the wandering k-space sampling. Susceptibility artifacts which reflect the spatial inhomogeneity of the magnetic field in the image primarily occur in the plug connection device due to the low bandwidth resulting from this. The magnetic field inhomogeneity is essentially caused by the subject itself or, respectively, by the spatially changing susceptibilities inside the subject (inside the body). An example of this is the boundary surface in the naso-tracheal area of a test subject between air and the skull or, respectively, the brain (air vs. bone/water).
The artifacts resulting from this are designated as a “static effect” because the adulterate the measurement result due to static inhomogeneities of the magnetic field (thus without gradient activities or patient movement).
In addition to the “static effects”, there are also dynamic influences on the image quality of the measurement due to temporal susceptibility changes in the subject region. For example, a slight rotation of the patient during a functional MR measurement can already distinctly adulterate the measurement or, respectively, the measurement result.
Local magnetic field changes likewise result due to the breathing of the patient (due to the changing air quantity in the lungs) and his pulse beat (brain pulsation). These circumstances also lead to a degradation of the image quality.
While “static effects” according to the prior art can be corrected by a number of methods—some methods for this utilize field maps acquired in advance, other methods are based on the use of reference scans—the problem of an optimal time-resolved correction of “dynamic effects” is not yet solved. Furthermore, either it presently assumed that the initial magnetic field inhomogeneities remain constant during the entire MR experiment (however, this is not provided given in vivo measurements due to continuous patient breathing, pulse beat, possible position change or movement of the head itself and/or of extremities) or time-intensive prescan-based correction methods are used that, however, entail a significant extension of the total measurement time and thus lead to an additional stressing of the patient.